This invention relates to a photovoltaic ultraviolet sensor comprising a zinc oxide single crystal.
JP-A H03-241777, which is incorporated herein by reference in its entirety, discloses a photoconductive ultraviolet sensor which comprises a substrate and a zinc oxide thin film formed on the substrate. In JP-A H03-241777, the zinc oxide thin film preferably has an optical forbidden band of about 3.0 to about 3.2 eV. However, the zinc oxide thin film of the foregoing optical forbidden band is sensitive also to visible rays and therefore has insufficient sensitivity to ultraviolet rays.
JP-A H10-182290, which is incorporated herein by reference in its entirety, discloses another ultraviolet sensor which comprises a zinc oxide crystal whose “a” face is used as an ultraviolet receiver surface. To sense ultraviolet rays, impedance variation of the zinc oxide crystal is monitored while the zinc oxide crystal is supplied with electric fields according to an antiresonance frequency of the zinc oxide crystal. However, generation of the antiresonance frequency requires special equipment so that the total cost of the ultraviolet sensor becomes high. In addition, the ultraviolet sensor of JP-A H10-182290 has a relatively slow response because of its sensing mechanism.